Pulse width modulation (PWM) is a method to control the root mean square voltage (VRMS) across loads by dividing a DC voltage into pulses. Varying the pulse width in a PWM period controls the VRMS across loads. To limit the electromagnetic interference (EMI) of a PWM system, the PWM pulses are not perfectly squared signals. EMI is reduced by having the rise and fall time of the pulses to be as long as possible. The long rise/fall times permit the switching components (SCs), such as FETs, to be turned on and off smoothly and reduce harmonic interference. This usually does not create a problem for a switching component even though power dissipation in the form of heat results from a prolonged turn on/turn off time. Heat sinks can be used in conjunction with the SCs to dissipate thermal effects. However, in high current applications, such as short circuit protection or a running motor condition, the small turn-off slew rate creates losses in the form of excessive heat, which can damage the switching component in the event of an abnormally high current cut-off. In such situations, a rapid turn off is the only way to protect the switching component.
Conventional PWM systems apply a fixed turn on/off slew rate such that either the EMI is reduced or the switching loss is reduced, but not both. An attempt to minimize both EMI and switching losses has generally been viewed as contradictory. Usually, EMI reduction receives higher priority. To counter the effects of switching losses when EMI reduction is optimized, PWM system designers have turned to larger switching components and larger heat sinks to handle unusually high current levels. The larger switching components and heat sinks require additional space and weight to be added to the PWM system. These larger components may also increase the price of the PWM system.
The present invention overcomes the above deficiencies by employing an adaptive switching speed control (ASC) for PWM. Different switching slew rates are used for different working modes, which include the normal mode where EMI reduction is given priority and an over-current mode, where reduction of switching losses is given priority. In the over-current mode, the invention further takes into consideration whether a start up operation condition exists to provide a third mode of operation. In the start up operation mode, the ASC PWM provides a semi-large switching slew rate and a certain amount of current inrush. In particular, for an automotive lighting system, a long start up time is not acceptable to bring an automotive lamp, such as a braking-indicator lamp, a turn signal-indicating lamp or a high beam lamp, into a luminous condition. To shorten this start up time, a certain amount of inrush current (e.g., 150%-200% of normal operating current) is needed during start up. However, inrush current is highly dependent on the turn-off slew rate of the control signal of a switching component. A fast turn off will result in a smaller inrush current during the start up period. Therefore, when a certain amount of inrush current is necessary for a start up operation of a lamp, the ASC PWM operates the switching component with a semi-large turn off slew rate to provide a certain amount of inrush current during the start up period. This transition only exists for certain PWM channels where a certain amount of inrush current is necessary. The present invention also provides an over-temperature mode as a fourth mode of operation. The over-temperature mode is characterized by an intermediate switching slew rate that expands the working temperature range of a device controlled by a PWM circuit in the event of high ambient temperatures. In the normal mode, the switching slew rates are small, providing a long switching transition to reduce EMI and detrimental harmonic effects. In the over-current mode, the switching slew rates for the falling edge are large, providing a short switching transition to reduce switching losses and avoid damage to the switching component. In the start up operation mode, the switching slew rate is slightly smaller than in the in over-current mode where no start up condition exists. In the over-temperature mode, an intermediate switching slew rate between that of the normal mode and the over-current modes is used.
Functionally, an adaptive switching speed control PWM (ASC PWM) system includes three sections: The controller and drivers, the power switches and loads, and the ASC controller. The present invention is directed to the ASC controller of the system. Since all components of the ASC controller are smaller power components, it is possible to integrate the ASC controller into a trigger driver of switch components, e.g., a high side N channel power MOSFET driver. This would provide an efficient implementation for a practical ASC PWM system. The integration is helpful to reduce the number of components in the system, reduce the size of the system and increase the system reliability as well.